1. Field of the Invention
The present invention relates to a thin-film transistor and a method for producing the same and a liquid crystal display device.
2. Description of the Related Art
Recently, a liquid crystal display device having an active matrix drive including thin-film transistors (TFT) has been remarkably developed. The active matrix includes TFTs and picture electrodes arranged in regions defined by intersecting gate bus lines and drain bus lines.
The TFT includes a gate electrode, a gate insulation film, an operative semiconductor film, a channel protection film, a source electrode and a drain electrode. An ohmic contact layer is formed on the operative semiconductor film, through which the source electrode and the drain electrode are connected to the operative semiconductor film. When the TFT is produced, the gate electrodes and the gate bus lines are integrally formed each other on the surface of the substrate and then above components are then layered thereon. The drain electrodes are formed together with the drain bus lines, and the source electrodes are connected to the picture electrodes. The source electrode and the drain electrode may be reversed to each other.
The source electrode and the drain electrode are disposed above the gate electrode via an insulation film and the operative semiconductor film, and, if the source electrode and the drain electrode are overlapped with the gate electrode as seen from above, a stray capacitance is created. Since the stray capacitance interferes with the image formation, it is preferable that the source electrode and the drain electrode are not overlapped with the gate electrode.
Japanese Unexamined Patent Publication (Kokai) No. 7-106585, for example, discloses an arrangement in that the source electrode and the drain electrode are not overlapped with the gate electrode so that the stray capacitance created therebetween is minimized. According to this Patent Publication, a channel protection film is formed in self-alignment with the gate electrode so that the source electrode and the drain electrode can be selectively formed while using this channel protection film.
Japanese Unexamined Patent Publication (Kokai) No. 6-204247 also discloses an arrangement in that, by the self-alignment and the selective film formation, the overlap of the source electrode and the drain electrode with the gate electrode can be eliminated to minimize the stray capacity created therebetween. Further in this Patent Publication, a second semiconductor film is formed on the operative semiconductor film, outside the channel protection film and the source electrode and the drain electrode are provided on the second semiconductor film by the selective film formation.
In the arrangement described in said Japanese Kokai 6-204247, the second semiconductor film operates as an ohmic contact to further enhance the electric connection between the operative semiconductor film and the source and drain electrodes. There is a problem, however, when the source and drain electrodes are formed on the second semiconductor film by a thermal CVD method or the like, in that the second semiconductor film tends to be eroded to deteriorate the contact characteristic. While it is possible to provide the ohmic contact by doping the outer portion of the operative semiconductor film with an impurity, there is another problem in such a case in that the doped portion of the semiconductor film is eroded when the source and drain electrodes are formed by the thermal CVD method.